Beamforming using an antenna array

ABSTRACT

There is provided mechanisms for transmitting a signal using a beamforming antenna array. A method being is by a network node. The method comprises obtaining an indication for transmission of the signal requiring use of a partial antenna array of the antenna array, the partial antenna array requiring less than all of the physical elements of the antenna array for transmission. The method comprises forming virtual antenna elements for the partial antenna array, thereby reducing the full antenna aperture of the antenna array. The method comprises expanding weight factors applied to the virtual antenna elements by connecting each of the virtual antenna elements to at least part of all physical antenna elements of the antenna array such that the virtual antenna elements at least partly utilize the full antenna aperture. The method comprises initiating transmission of the signal using the array of the virtual antenna elements.

RELATED APPLICATIONS

This application is a continuation of prior application Ser. No.15/548,464, filed 3 Aug. 2017, which was the National Stage ofInternational Application PCT/EP2016/054622 filed 4 Mar. 2016, whichclaims priority to PCT/EP2015/054783 filed 6 Mar. 2015, the entiredisclosure of each being hereby incorporated by reference herein.

TECHNICAL FIELD

Embodiments presented herein relate to a method, a network node, acomputer program, and a computer program product for transmitting asignal using a beamforming antenna array.

BACKGROUND

In communications networks, there may be a challenge to obtain goodperformance and capacity for a given communications protocol, itsparameters and the physical environment in which the communicationsnetwork is deployed.

One component of wireless communications networks where it may bechallenging to obtain good performance and capacity is the antennas ofnetwork nodes configured for wireless communications; either to/fromanother network node, and/or to/from a wireless user terminal.

For example, massive beamforming, i.e., beamforming using active antennaarrays with orders of magnitudes more antenna elements than used incurrent communications networks, is expected to become a technicalcomponent in the radio access part of future fifth generation (5G)communications networks. By using large antenna arrays at the radio basestations, user data can be transmitted focused in space so that energyis received mainly by the wireless device dedicated by the user data,thus resulting in little interference being perceived by other wirelessdevices or other types of nodes. Massive beamforming has therefore thepotential to increase system capacity and energy efficiency by orders ofmagnitudes.

As carrier frequency increases it is foreseen that the number of radiochains in the active antenna arrays can be comparatively large, possiblyseveral hundreds. This type of large antenna arrays is suitable foranalog beamforming networks since it does not need any attenuators. Thistype of large antenna arrays is also suitable for user specificbeamforming by creating narrow transmission beams in directions wherethe radio channel is strong towards a user. However, when signals are tobe transmitted over a larger area, for example control planetransmission, the antenna arrays are not that suitable if conventionaltechniques are used to form the wider transmission beams. Theconventional technique is to apply amplitude taper which results in manypower amplifiers in the radio chains being more or less not used and, asa consequence, the total output power is reduced.

Hence, there is still a need for improved beamforming mechanisms.

SUMMARY

An object of embodiments herein is to provide efficient beamformingmechanisms.

According to a first aspect there is presented a method for transmittinga signal using a beamforming antenna array. The method is performed by anetwork node. The method comprises obtaining an indication fortransmission of the signal requiring use of a partial antenna array ofthe antenna array, the partial antenna array requiring less than all ofthe physical elements of the antenna array for transmission. The methodcomprises forming virtual antenna elements for the partial antennaarray, thereby reducing the full antenna aperture of the antenna array.The method comprises expanding weight factors applied to the virtualantenna elements by connecting each of the virtual antenna elements toat least part of all physical antenna elements of the antenna array suchthat the virtual antenna elements at least partly utilize the fullantenna aperture. The method comprises initiating transmission of thesignal using the array of the virtual antenna elements.

Advantageously this method provides efficient beamforming of the signalto be transmitted using the antenna array.

Advantageously this methods enables an efficient antenna architecture tobe used for transmitting a signal with an adjustable beam width.

The beam width can be very wide compared to the beam width correspondingto the full antenna array size, even as wide as for a single antennaelement.

All power amplifiers of the antenna array can be fully utilized, i.e.with only phase taper applied.

Advantageously, this method enables a desired number of antenna ports tobe defined, the number of antenna ports being less than, or equal to,the number of physical antenna elements of the antenna array.

The antenna architecture can be based on either linear (1-D) or planar(2-D) antenna arrays.

According to a second aspect there is presented a network node fortransmitting a signal using a beamforming antenna array. The networknode comprises processing circuitry. The processing circuitry isconfigured to cause the network node to obtain an indication fortransmission of the signal requiring use of a partial antenna array ofthe antenna array, the partial antenna array requiring less than all ofthe physical elements of the antenna array for transmission. Theprocessing circuitry is configured to cause the network node to formvirtual antenna elements for the partial antenna array, thereby reducingthe full antenna aperture of the antenna array. The processing circuitryis configured to cause the network node to expand weight factors appliedto the virtual antenna elements by connecting each of the virtualantenna elements to at least part of all physical antenna elements ofthe antenna array such that the virtual antenna elements at least partlyutilize the full antenna aperture. The processing circuitry isconfigured to cause the network node to initiate transmission of thesignal using the array of the virtual antenna elements.

According to a third aspect there is presented a network node fortransmitting a signal using a beamforming antenna array. The networknode comprises processing circuitry and a computer program product.Computer program product stores instructions that, when executed by theprocessing circuitry, causes the network node to perform operations, orsteps. The operations, or steps, cause the network node to obtain anindication for transmission of the signal requiring use of a partialantenna array of the antenna array, the partial antenna array requiringless than all of the physical elements of the antenna array fortransmission. The operations, or steps, cause the network node to formvirtual antenna elements for the partial antenna array, thereby reducingthe full antenna aperture of the antenna array. The operations, orsteps, cause the network node to expand weight factors applied to thevirtual antenna elements by connecting each of the virtual antennaelements to at least part of all physical antenna elements of theantenna array such that the virtual antenna elements at least partlyutilize the full antenna aperture. The operations, or steps, cause thenetwork node to initiate transmission of the signal using the array ofthe virtual antenna elements.

According to a fourth aspect there is presented network node fortransmitting a signal using a beamforming antenna array. The networknode comprises an obtain module configured to obtain an indication fortransmission of the signal requiring use of a partial antenna array ofthe antenna array, the partial antenna array requiring less than all ofthe physical elements of the antenna array for transmission. The networknode comprises a form module (101 b) configured to form virtual antennaelements for the partial antenna array, thereby reducing the fullantenna aperture of the antenna array. The network node comprises anexpand module configured to expand weight factors applied to the virtualantenna elements by connecting each of the virtual antenna elements toat least part of all physical antenna elements of the antenna array suchthat the virtual antenna elements at least partly utilize the fullantenna aperture. The network node comprises an initiate moduleconfigured to initiate transmission of the signal using the array of thevirtual antenna elements.

According to a fifth aspect there is presented a computer program fortransmitting a signal using a beamforming antenna array, the computerprogram comprising computer program code which, when run on a networknode, causes the network node to perform a method according to the firstaspect.

According to a sixth aspect there is presented a computer programproduct comprising a computer program according to the fifth aspect anda computer readable storage medium on which the computer program isstored.

It is to be noted that any feature of the first, second, third, fourth,fifth and sixth aspects may be applied to any other aspect, whereverappropriate. Likewise, any advantage of the first aspect may equallyapply to the second, third, fourth, fifth, and/or sixth aspect,respectively, and vice versa. Other objectives, features and advantagesof the enclosed embodiments will be apparent from the following detaileddisclosure, from the attached dependent claims as well as from thedrawings.

Generally, all terms used in the claims are to be interpreted accordingto their ordinary meaning in the technical field, unless explicitlydefined otherwise herein. All references to “a/an/the element,apparatus, component, means, step, etc.” are to be interpreted openly asreferring to at least one instance of the element, apparatus, component,means, step, etc., unless explicitly stated otherwise. The steps of anymethod disclosed herein do not have to be performed in the exact orderdisclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, withreference to the accompanying drawings, in which:

FIGS. 1, 2, 3, 4, 5, and 6 are schematic diagrams illustrating differentaspects of antenna arrays according to embodiments;

FIGS. 7 and 8 are flowcharts of methods according to embodiments;

FIG. 9 is a block diagram showing functional units of a network nodeaccording to an embodiment;

FIG. 10 is a block diagram showing functional modules of a network nodeaccording to an embodiment;

FIG. 11 schematically illustrates a radio access network node accordingto an embodiment;

FIG. 12 schematically illustrates a wireless device according toembodiments; and

FIG. 13 schematically illustrates a computer program product accordingto an embodiment.

DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter withreference to the accompanying drawings, in which certain embodiments ofthe inventive concept are shown. This inventive concept may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided by way of example so that this disclosure will be thorough andcomplete, and will fully convey the scope of the inventive concept tothose skilled in the art. Like numbers refer to like elements throughoutthe description. Any step or feature illustrated by dashed lines shouldbe regarded as optional.

The herein proposed antenna array and method offers both beam patternswith desired beam shapes, as well as excellent power utilization. Theembodiments disclosed herein particularly relate to transmitting asignal using a beamforming antenna array. In order to obtain suchtransmitting there is provided a network node, a method performed by thenetwork node, a computer program product comprising code, for example inthe form of a computer program, that when run on a network node, causesthe network node to perform the method.

FIG. 1 is a schematic block diagram illustrating an example architectureof a two dimensional antenna array 1 for which embodiments presentedherein can be applied. However, the embodiments presented herein areequally applicable to one-dimensional antenna arrays. The antenna array1 can thus either be a linear array (1-D), a uniform linear array (ULA),or planar array (2-D), uniform rectangular array (URA).

The antenna front end comprises a physical array 1 e comprising antennaelements where each antenna element may be a sub-array of severalradiating antenna elements connected via feed networks to two subarrayports having orthogonal polarization. Each subarray port is connected toa radio chain as comprised in a radio array 1 d. The number of subarrayports in block 1 b accessible to baseband signal processing may bereduced via a port reduction block 1 c that creates new antenna portsthat are (linear) combinations of the input antenna ports. Access ismade to the subarray ports at baseband if both dedicated and broadcasteddata is to be transmitted at the same time. Further, in general terms,access to all subarray ports may be needed for shaping wide beamsaccording to the herein disclosed mechanisms for beamforming. In thebaseband signal processing block 1 a virtual antenna ports may becreated by matrix multiplications. These virtual antenna ports may be ofdifferent type. For example, in Long Term Evolution (LTE) radio accesssystems they may for a radio base station carry common reference signals(CRS) at ports 0-3, channel state information reference signals (CSI-RS)at port 15-22, and user-specific reference signals and data at ports7-14. In some implementations one or several blocks of the in the twodimensional antenna array 1 in FIG. 1 may be removed.

FIG. 2 is a schematic block diagram illustrating a possibleimplementation of the two dimensional antenna array 1 of FIG. 1. Itcomprises a beam former comprising blocks 1 a, 1 b, 1 c of FIG. 1, aradio array 1 d and a physical antenna array 1 e. In the example of FIG.2 there are two antenna ports per subarray. The beam former 1 a-c isconfigured to receive user data and control data, beamforming weightsfor the user data, beamforming weights for reference signals, such asCSI-RS and beamforming weights for wide beam transmission.

FIG. 3 at (a) schematically illustrates a physical antenna array 1 e ofan antenna array 1 of size (N_(x)×M_(x)) comprising physical antennaelements, one of which is identified at reference numeral 31. FIG. 3 at(b) schematically illustrates a virtual antenna array 1 e′ comprising ofsize (N_(z)×M_(z)) and virtual antenna elements, one of which isidentified at reference numeral 31′, where the virtual antenna elementsare separated a distance dr_(M) in dimension M and a distance dr_(N) indimension N. In this respect, the virtual antenna array can be regardedas a subset of the antenna array 1 with respect to power patterns, butnot polarizations, for the antenna elements; the antenna elements of thephysical antenna array 1 e have the same power patterns. Each antennaelement of the virtual antenna array is mapped to a multitude of antennaelements in the physical antenna array 1 e. Embodiments of how toimplement this mapping will be disclosed below.

FIG. 4 schematically illustrates reduction (from right to left) andexpansion (from left to right) a factor 2 via weight factors for antennaelements, two of which are identified at reference numerals 41, 41′, intwo physical antenna arrays 1 e, 1 e′. According to the illustrativeexample of FIG. 4, each weight factor takes only a value in the setdefined by the values {−1,+1}. Thus, the antenna elements 41 of thephysical antenna array 1 e can be expanded to the antenna elements 41′of the physical antenna array 1 e′ and the antenna elements 41′ of thephysical antenna array 1 e′ can be reduced to the antenna elements 41 ofthe physical antenna arrays 1 e. Embodiments for how to perform such anexpansion and such a reduction will be disclosed below under aperturereduction and aperture expansion, respectively.

FIG. 5 schematically illustrates two embodiments of a physical antennaarray 1 e. Each physical antenna array 1 e comprises antenna elements51, 54, where each antenna element comprises two sub-elements 52, 53,55, 56 having orthogonal polarizations in all directions (of interest).In FIG. 5(a) the two sub-elements 52, 53 are located at the sameposition, and in FIG. 5(b) the two sub-elements 55, 56 are displaced inrelation to each other, but still considered to be part of the sameantenna element 54. Sub-elements part of the same antenna element form asub-element pair. In FIG. 5(5) the sub-elements 55 and 56 thus form asub-element pair. In relation thereto, the displacement is the same forall sub-element pairs.

One reason for using all antenna elements in the antenna array, as wellas for the subarray port mapping, and thus beam port mapping, to haveuniform amplitude, is to efficiently use the available power resource.This applies specifically for an active antenna array with distributedpower amplifiers but it also applies for an antenna array with a powerdistribution network 60 a, 60 b comprising phase shifters 62, possiblyalso attenuators 61 as in FIGS. 6(a) and 6(b).

FIGS. 7 and 8 are flow chart illustrating embodiments of methods fortransmitting a signal using a beamforming antenna array 1. The methodsare performed by the network node 100. The methods are advantageouslyprovided as computer programs 320.

Reference is now made to FIG. 7 illustrating a method for transmitting asignal using a beamforming antenna array 1 as performed by the networknode 100 according to an embodiment.

The method for transmitting a signal using a beamforming antenna array 1is based on starting from the full aperture of the antenna array 1 andcombining the physical antenna elements of the antenna array 1 to formnew virtual antenna elements.

S102: The network node 100 obtains an indication for transmission of thesignal. The transmission of the signal requires use of a partial antennaarray of the antenna array. The partial antenna array requires less thanall of the physical elements of the antenna array for transmission. Inother words, the partial antenna array requires use of less than all ofthe physical elements of the antenna array for transmission of thesignal.

The method for transmitting a signal using a beamforming antenna array 1is based on starting from the full aperture of the antenna array 1 andcombining the physical antenna elements of the antenna array 1 to formnew virtual antenna elements, as in steps S104, S106:

S104: The network node 100 forms virtual antenna elements for thepartial antenna array. The full antenna aperture of the antenna array isthereby reduced.

The virtual antenna elements have the same power pattern as the physicalantenna elements but the power resource is much higher (actually the sumof the power resource for all antenna elements being combined). Thenetwork node 100 thus expands the virtual antenna elements to at leastpartly utilize the full antenna aperture, as in step S106:

S106: The network node 100 expands weight factors applied to the virtualantenna elements. The weight factors are expanded by the network node100 connecting each of the virtual antenna elements to at least part ofall physical antenna elements of the antenna array such that the virtualantenna elements at least partly utilize the full antenna aperture.

The combination of virtual antenna elements is repeated until theresulting number of virtual antenna elements, i.e., the virtualaperture, is as desired. Transmission of the signal is then initiated,as in step S108:

S108: The network node 100 initiates transmission of the signal usingthe array of the virtual antenna elements.

A feed network is thereby defined such that the physical antenna array Xof dual polarized antenna elements of size (N_(x)×M_(x)) is reduced to avirtual antenna array, Z, comprising (N_(z)×M_(z)) virtual antennaelements. The feed network can be expressed in terms of weight factorsin weighting matrices. The elements in this virtual antenna array Z canbe combined to form one or more beam ports.

According to some embodiments the angular power pattern of the virtualantenna elements is identical to the angular power pattern of one of thephysical antenna elements of the antenna array. Some properties of thefeed network are such that the angular power spectrum for the fullphysical aperture of the physical antenna array X is identical to thepower spectra of the antenna aperture defined by the virtual antennaarray, Z. Hence, according to embodiments the weight factors areexpanded such that the virtual antenna elements utilize the full antennaaperture. Further, the virtual antenna elements have a total power beingthe sum of output power for all physical antenna elements of the antennaarray.

According to some embodiments the indication is for a needed beam fortransmission of the signal. The needed beam requires less than all ofthe physical elements of the antenna array for transmission. Forexample, the needed beam could be defined by a needed beam width or aneeded number of antenna ports for transmission of the signal using theantenna array 1. In other words, by means of the network node 100performing steps S102-S108 it is possible to transmit the signal in atransmission beam where the transmission beam utilizes a low number ofvirtual antenna elements, typically meaning large beam widths, via thelarge full physical aperture, X, and still get the same angular powerspectra, i.e. beam shape. This enables all power amplifiers in an activeantenna array to be used, regardless of beam width.

Embodiments relating to further details of transmitting a signal using abeamforming antenna array 1 will now be disclosed.

Reference is now made to FIG. 8 illustrating methods for transmitting asignal using a beamforming antenna array 1 as performed by the networknode 100 according to further embodiments. Steps S102-S108 are performedas disclosed with reference to FIG. 7 and a repetition of these steps istherefore omitted.

Aperture Reduction

Operations of converting the physical antenna array X to the virtualantenna array Z define embodiments of how the virtual antenna elementscan be formed in step S104 and will now be described as a two-stepprocess. In a first reduction step the full aperture of the antennaarray 1 is reduced along a first dimension, which in this description isselected as the first, or the “M”, dimension.

The physical antenna array X can be regarded as a matrix describing theexcitation in the aperture of the antenna elements with polarization A,X_(A), and polarization B, X_(B), respectively:

${X = \begin{bmatrix}X_{A} \\X_{B}\end{bmatrix}},{X \in C^{2N_{x}{xM}_{x}}}$

The first step of converting the physical antenna array X to the virtualantenna array Z comprises determining a matrix {tilde over (Y)} asfollows:

$\overset{\sim}{Y} = {\begin{bmatrix}{\overset{\sim}{Y}}_{A} \\{\overset{\sim}{Y}}_{B}\end{bmatrix} = {Q^{H}\begin{bmatrix}X_{A}^{T} \\X_{B}^{T}\end{bmatrix}}}$

Here, Q is a matrix describing the reduction of elements as follows:Q=[Q _(2A) Q _(2B)], where Q _(2A) and Q _(2B) ∈C ^(2M) ^(x) ^(×M) ^(z)

In other words, according to embodiments, Q comprises a first factorQ_(2A) for reduction along a first dimension and resulting in a firstpolarization of the virtual antenna elements and a second factor Q_(2B)for reduction along a first dimension and resulting in a secondpolarization of the virtual antenna elements.

Further, Y is a matrix describing the excitation of a first virtualantenna array and is found from {tilde over (Y)} as follows:

${Y = {\begin{bmatrix}Y_{A} \\Y_{B}\end{bmatrix} = \begin{bmatrix}{\overset{\sim}{Y}}_{A}^{T} \\{\overset{\sim}{Y}}_{B}^{T}\end{bmatrix}}},{Y \in C^{2N_{x} \times M_{z}}}$

The reduction along the “N”-dimension takes place in a second reductionstep resulting in a virtual aperture Z as follows:

$Z = {{R^{H}Y} = {R^{H}\begin{bmatrix}Y_{A} \\Y_{B}\end{bmatrix}}}$

Here, R is a matrix describing the reduction of elements.

Hence, according to embodiments the virtual antenna elements are formedby reduction factors Q and R being applied to all antenna elements ofthe antenna array 1.

Two matrixes R_(2A) (for 2A to be read as to-A, i.e. transformation toan element with polarization A) and R_(2B) (for 2B to be read as to-B,i.e. transformation to an element with polarization B) are defined asfollows:R=[R _(2A) R _(2B)], where R _(2A) and R _(2B) ∈C ^(2N) ^(x) ^(×N) ^(z)

In other words, according to embodiments R comprises a first factorR_(2A) for reduction along the second dimension and resulting in thefirst polarization of the virtual antenna elements and a second factorR_(2B) for reduction along the second dimension and resulting in thesecond polarization of the virtual antenna elements.

How to determine the reduction matrices Q and R to achieve identicalpower spectra (or power patterns) when transmitting over the antennaaperture Z and the antenna aperture X will be disclosed below. Examplesof such matrices Q and R resulting in a reduction of the full aperturewith a factor of 2, 6 and 10 will now be disclosed.

Reduction Factor 2

A vector u₂ ^(T) is defined as:u ₂ ^(T)=[u _(2A) T u _(2B) ^(T)], where u _(2A) ^(T) and u _(2B) ∈C^(2×1)

Vectors u_(2A) ^(T) and u_(2B) ^(T) are defined as follows:u _(2A) ^(T)=[e ^(iδ) ¹ 0]/√{square root over (2)}u _(2B) ^(T)=[e ^(iδ) ² 0]/√{square root over (2)}

Here, δ₁ and δ₂ denote phase angles and can be chosen arbitrarily.

A vector v₂ is derived from vectors u_(2A) and u_(2B) in the followingway to ensure identical power pattern and orthogonal polarization:v ₂ ^(T) =e ^(iδ) ³ [u _(2B) ^(T) F−u _(2A) ^(T) F]*

Here, F is a matrix that reverses the order of the elements in thevector it operates on and [ ]* denotes complex conjugate of the elementit operates on. Further, δ₃ denotes a phase angle and can be chosenarbitrarily. Hence, according to embodiments the elements of Q_(2B) arederived from Q_(2A) by reversing in order, negating, and/or complexconjugating the elements of Q_(2A). Similarly, the elements of R_(2B)are derived from R_(2A) by reversing in order, negating, and/or complexconjugating the elements of R_(2A).

The two matrices R_(2A) and R_(2B) are determined as follows:

R_(2A) = u₂ ⊗ I_(N_(x/r))  and  R_(2B) = v₂ ⊗ I_(N_(x/r))

Here, “⊗” denotes the Kronecker product,

I_(N_(x/r))denotes an identity matrix of size N_(x)/r, where N_(x) is the dimensionof the aperture at hand to be reduced, and r is the reduction factor (inthis case r=2). The matrix Q is found in a similar way.

Some properties to consider when determining the matrices R_(2A),R_(2B), Q_(2A) and Q_(2B) will now be summarized.

The corresponding spatial power spectra are white. The elements withnon-zero magnitude are subject to phase-only tapering. Hence, accordingto embodiments all non-zero elements of the reduction factors Q and Rhave constant modulus.

The matrices R_(2A) and R_(2B) result in virtual antenna ports withorthogonal polarization in all directions. The matrices R_(2A) andR_(2B) have the same reduction factor.

The matrices Q_(2A) and Q_(2B) result in virtual antenna ports withorthogonal polarization in all directions. The matrices Q_(2A) andQ_(2B) have the same reduction factor.

Given that the antenna array consists of antenna elements withorthogonal polarizations, so the same will apply also for the virtualarray.

Hence, according to embodiments, the matrices matrices Q_(2A) and Q_(2B)have identical dimensions resulting in the same reduction, and thematrices R_(2A) and R_(2B) have identical dimensions resulting in samethe reduction.

All elements in the reduced antenna array, with a given polarization,are the result from applying the same reduction matrices, except from atranslation.

The separation between antenna elements (phase centers) in the reducedaperture is the same as for the non-reduced antenna array. Hence,according to embodiments the virtual antenna elements for eachpolarization have a phase center separation identical to the phasecenter separation of the full antenna aperture.

Reduction Factor 6

For a reduction factor of six, the vectors u₆ ^(T), u_(6A) ^(T), andu_(6B) ^(T) are defined as follows:

u₆^(T) = [u_(6A)^(T)  u_(6B)^(T)], where  u_(6A)^(T)  and  u_(6B)^(T) ∈ C^(6 × 1)and$u_{6A}^{T} = {{e^{i\;\delta_{1}}\left\lbrack {e^{i\frac{3\pi}{4}}\mspace{14mu} e^{i\; 0}\mspace{14mu} e^{i\frac{\pi}{4}}\mspace{14mu} 0\mspace{14mu} 0\mspace{14mu} 0} \right\rbrack}\text{/}\sqrt{6}}$$u_{6B}^{T} = {{e^{i\;\delta_{2}}\left\lbrack {e^{i\frac{\pi}{4}}\mspace{14mu} e^{i\; 0}\mspace{14mu} e^{i\frac{3\pi}{4}}\mspace{14mu} 0\mspace{14mu} 0\mspace{14mu} 0} \right\rbrack}\text{/}\sqrt{6}}$

The phase angles δ₁ and δ₂ can be arbitrarily chosen and a linearphaseshift can be applied over u_(6A) ^(T) and u_(6B) ^(T) as long asthe linear phaseshift is the same for both vectors u_(6A) ^(T) andu_(6B) ^(T).

The vector v₆ can be derived from the vectors u_(6A) and u_(6B) in thefollowing way to ensure identical power pattern and orthogonalpolarization:v ₆ ^(T) =e ^(iδ) ³ [u _(6B) ^(T) F−u _(6A) ^(T) F]*

As above, F is a matrix that reverses the order of the elements in thevector it operates on and [ ]* denotes complex conjugate of the elementit operates on. The phase angle δ₃ can be chosen arbitrarily.

The two matrixes R_(2A) and R_(2B) are then determined as follows:

R_(2A) = u₆ ⊗ I_(N_(x/r))  and  R_(2B) = v₆ ⊗ I_(N_(x/r))

As above, I_(N) _(x) /r denotes an identity matrix of size N_(x)/r,where N_(x) is the dimension of the aperture at hand to be reduced and ris the reduction factor (in this case r=6). Q is, as above, found in asimilar way.

Reduction Factor 10

Yet another reduction factor is 10 for which the vectors u_(10A) ^(T)and u_(10B) ^(T) are defined as follows:

$u_{10A}^{T} = {{e^{i\;\delta_{1}}\left\lbrack {e^{{- i}\frac{2\pi}{4}}\mspace{14mu} e^{{- i}\frac{\pi}{4}}\mspace{14mu} e^{i\; 0}\mspace{14mu} e^{i\frac{3\pi}{4}}\mspace{14mu} e^{i\; 0}\mspace{14mu} 0\mspace{14mu} 0\mspace{14mu} 0\mspace{14mu} 0\mspace{14mu} 0} \right\rbrack}\text{/}\sqrt{10}}$$u_{10B}^{T} = {{e^{i\;\delta_{2}}\left\lbrack {e^{i\; 0}\mspace{14mu} e^{i\frac{3\pi}{4}}\mspace{14mu} e^{i\; 0}\mspace{14mu} e^{{- i}\frac{\pi}{4}}\mspace{14mu} e^{{- i}\frac{2\pi}{4}}\mspace{14mu} 0\mspace{14mu} 0\mspace{14mu} 0\mspace{14mu} 0\mspace{14mu} 0} \right\rbrack}\text{/}\sqrt{10}}$

Derivation of R_(2A) and R_(2B) is performed as for reduction factor sixdisclosed above and not shown here.

Iterative Reduction

In case reduction factors two, six and ten are not sufficient, thesereduction factors can be combined in any desired way. Let R_(tot) definea total reduction matrix as follows:

$R_{tot} = {\underset{k}{\Pi}{R_{k}\left( {N_{k},r_{k}} \right)}}$

Here, R(N_(k), r_(k)) is reduction matrix for stage k and r_(k) is thereduction for stage k. N_(k) is found from the previous stage asN_(k-1)/r_(k-1).

As an example, the sizes for the matrices R_(tot)=R₁R₂R₃ becomes

${\left( {2N_{1} \times \frac{2N_{1}}{r_{1}}} \right)\left( {\frac{2N_{1}}{r_{1}} \times \frac{2N_{1}}{r_{1}r_{2}}} \right)\left( {\frac{2N_{1}}{r_{1}r_{2}} \times \frac{2N_{1}}{r_{1}r_{2}r_{3}}} \right)} = {\left( {2N_{1} \times \frac{2N_{1}}{r_{1}r_{2}r_{3}}} \right).}$As the skilled person understands, the matrix Q can be found in asimilar way. Hence, according to embodiments the full antenna apertureof the antenna array 1 is iteratively reduced at least two times alongat least one dimension of the antenna array 1.

Use of R and Q for Array Expansion

Next will be disclosed operations of how the matrices R and Q can usedfor enabling a large aperture of the antenna array to be used fortransmission beams that only require a small aperture for transmission.These operations define embodiments of how the weight factors can beexpanded in step S106. According to embodiments the network node 100 isconfigured to perform steps S106 a-s106 c in order to expand the partialantenna aperture.

S106 a: The network node 100 determines the weight factors for a weightmatrix W_(v). A weight matrix (also denoted an excitation matrix)

$W_{v} = \begin{bmatrix}W_{vA} \\W_{vB}\end{bmatrix}$is thus obtained for the desired beam shape to be used for transmissionof the signal. This matrix W_(v) corresponds to the small virtualantenna aperture Z and is then expanded to a large aperture in atwo-step procedure.

S106 b: The network node 100 expands the weight matrix W_(v) such thatthe virtual antenna elements at least partly utilize the full antennaaperture. According to embodiments the weight matrix W_(v) is expandedusing the matrices Q and R. This expansion will be described next.

The virtual antenna aperture is first expanded with matrix Q accordingto

$\begin{bmatrix}W_{vpA}^{T} \\W_{vpB}^{T}\end{bmatrix} = {Q\begin{bmatrix}W_{vA}^{T} \\W_{vB}^{T}\end{bmatrix}}$

Elements in the resulting matrix

$\begin{bmatrix}W_{vpA}^{T} \\W_{vpB}^{T}\end{bmatrix}\quad$(expanded along one dimension) are rearranged as follows:

$W_{vp} = \begin{bmatrix}W_{vpA} \\W_{vpB}\end{bmatrix}$

Finally the, virtual antenna aperture is expanded also in the orthogonaldimension to form the full aperture of the antenna array X as follows:

$W_{p} = {\begin{bmatrix}W_{pA} \\W_{pB}\end{bmatrix} = {RW}_{vp}}$

S106 c: The network node 100 applies the weight matrix to the virtualantenna elements.

Although at least some of the above disclosed embodiments relate toreduction and expansion being performed along two dimensions of theantenna array 1, the herein disclosed embodiments are equally applicablefor reduction and expansion being performed along one dimension of theantenna array 1. However, alternatively, the herein disclosed reductionand expansion is indeed performed along both dimensions of the antennaarray 1.

FIG. 9 schematically illustrates, in terms of a number of functionalunits, the components of a network node 100 according to an embodiment.Processing circuitry 101 is provided using any combination of one ormore of a suitable central processing unit (CPU), multiprocessor,microcontroller, digital signal processor (DSP), etc., capable ofexecuting software instructions stored in a computer program product 130(as in FIG. 13), e.g. in the form of a storage medium 103. Theprocessing circuitry 101 may further be provided as at least oneapplication specific integrated circuit (ASIC), or field programmablegate array (FPGA).

Particularly, the processing circuitry 101 is configured to cause thenetwork node 100 to perform a set of operations, or steps, S102-S108, asdisclosed above. For example, the storage medium 103 may store the setof operations, and the processing circuitry 101 may be configured toretrieve the set of operations from the storage medium 103 to cause thenetwork node 100 to perform the set of operations. The set of operationsmay be provided as a set of executable instructions.

Thus the processing circuitry 101 is thereby arranged to execute methodsas herein disclosed. The storage medium 103 may also comprise persistentstorage, which, for example, can be any single one or combination ofmagnetic memory, optical memory, solid state memory or even remotelymounted memory. The network node 100 may further comprise acommunications interface 102 comprising an antenna arrangement 1. Theprocessing circuitry 101 controls the general operation of the networknode 100 e.g. by sending data and control signals to the communicationsinterface 102 and the storage medium 103, by receiving data and reportsfrom the communications interface 102, and by retrieving data andinstructions from the storage medium 103. Other components, as well asthe related functionality, of the network node 100 are omitted in ordernot to obscure the concepts presented herein.

FIG. 10 schematically illustrates, in terms of a number of functionalmodules, the components of a network node 100 according to anembodiment. The network node 100 of FIG. 10 comprises a number offunctional modules; an obtain module 101 a configured to perform stepS102, a form module 101 b configured to perform step S104, an expandmodule 101 c configured to perform step S106, and an initiate module 101d configured to perform step S108. The network node 100 of FIG. 10 mayfurther comprises a number of optional functional modules, such as anyof a determine module 101 e configured to perform step S106 a, an expandmodule 101 f configured to perform step S106 b, and an apply module 101g configured to perform step S106 c.

In general terms, each functional module 101 a-101 g may in oneembodiment be implemented only in hardware or and in another embodimentwith the help of software, i.e., the latter embodiment having computerprogram instructions stored on the storage medium 103 which when run onthe processing circuitry 101 makes the network node 100 perform thecorresponding steps mentioned above in conjunction with FIGS. 7 and 8.It should also be mentioned that even though the modules 101 a-101 gcorrespond to parts of a computer program, they do not need to beseparate modules therein, but the way in which they are implemented insoftware is dependent on the programming language used. Preferably, oneor more or all functional modules 101 a-101 g may be implemented by theprocessing circuitry 101, possibly in cooperation with functional units102 and/or 103. The processing circuitry 101 may thus be configured tofrom the storage medium 103 fetch instructions as provided by afunctional module 101 a-101 g and to execute these instructions, therebyperforming any steps as disclosed herein.

The antenna array 1 and/or the network node 100 may be provided asintegrated circuits, as standalone devices or as a part of a furtherdevice. For example, the antenna array 1 and/or network node 100 may beprovided in a radio transceiver device, such as in a radio accessnetwork node 110 or a wireless device 120. FIG. 11 illustrates a radioaccess network node 110 comprising at least one antenna array 1 and/ornetwork node 100 as herein disclosed. The radio access network node 110may be a BTS, a NodeB, an eNB, a repeater, a backhaul node, or the like.FIG. 12 illustrates a wireless device 120 comprising at least oneantenna array 1 and/or network node 100 as herein disclosed. Thewireless device 120 may be a user equipment (UE), a mobile phone, atablet computer, a laptop computer, etc. or the like.

The antenna array 1 and/or network node 100 may be provided as anintegral part of the further device. That is, the components of theantenna array 1 and/or network node 100 may be integrated with othercomponents of the further device; some components of the further deviceand the antenna array 1 and/or network node 100 may be shared. Forexample, if the further device as such comprises processing circuitry,this processing circuitry may be configured to perform the actions ofthe processing circuitry 101 of the network node 100. Alternatively theantenna array 1 and/or network node 100 are provided as separate unitsin the further device.

FIG. 13 shows one example of a computer program product 130 comprisingcomputer readable storage medium 132. On this computer readable storagemedium 132, a computer program 131 can be stored, which computer program131 can cause the processing circuitry 101 and thereto operativelycoupled entities and devices, such as the communications interface 102and the storage medium 103, to execute methods according to embodimentsdescribed herein. The computer program 131 and/or computer programproduct 130 may thus provide means for performing any steps as hereindisclosed.

In the example of FIG. 13, the computer program product 130 isillustrated as an optical disc, such as a CD (compact disc) or a DVD(digital versatile disc) or a Blu-Ray disc. The computer program product130 could also be embodied as a memory, such as a random access memory(RAM), a read-only memory (ROM), an erasable programmable read-onlymemory (EPROM), or an electrically erasable programmable read-onlymemory (EEPROM) and more particularly as a non-volatile storage mediumof a device in an external memory such as a USB (Universal Serial Bus)memory or a Flash memory, such as a compact Flash memory. Thus, whilethe computer program 131 is here schematically shown as a track on thedepicted optical disk, the computer program 131 can be stored in any waywhich is suitable for the computer program product 130.

The inventive concept has mainly been described above with reference toa few embodiments. However, as is readily appreciated by a personskilled in the art, other embodiments than the ones disclosed above areequally possible within the scope of the inventive concept, as definedby the appended patent claims. For examples, although using LTE specificterminology, the herein disclosed embodiments may also be applicable tocommunications networks not based on LTE, mutatis mutandis.

What is claimed is:
 1. A wireless device for transmitting a signal usinga beamforming antenna array, the wireless device comprising: processingcircuitry; memory containing instructions executable by the processingcircuitry whereby the wireless device is operative to: obtain anindication for transmission of the signal requiring use of less than allof the physical antenna elements of the antenna array for transmissionof the signal; reducing the full antenna aperture of the antenna arrayby forming virtual antenna elements for the less than all of thephysical antenna elements of the antenna array connecting each of thevirtual antenna elements to at least part of all physical antennaelements of the antenna array such that the virtual antenna elements atleast partly utilize the full antenna aperture; and initiatetransmission of the signal using the array of the virtual antennaelements.
 2. The wireless device of claim 1, wherein the virtual antennaelements form a virtual antenna array being a subset of the full antennaarray with respect to power patterns.
 3. The wireless device of claim 1wherein power amplifiers of the beamforming antenna array are fullyutilized.
 4. The wireless device of claim 1 wherein the beamformingantenna array has an antenna architecture based on one-dimensional ortwo-dimensional antenna arrays.
 5. A network node for transmitting asignal using a beamforming antenna array, the antenna array comprisingphysical antenna elements of a first polarization and physical antennaelements of a second polarization, and the signal requiring use of apartial antenna array of the antenna array, the partial antenna arrayrequiring less than all of the physical antenna elements of the antennaarray for transmission of the signal, the network node comprising:processing circuitry; memory containing instructions executable by theprocessing circuitry whereby the network node is operative to: form atleast one virtual antenna element for the partial antenna array tocreate a virtual aperture smaller than the full antenna aperture of theantenna array; connect each of the at least one virtual antenna elementto at least one physical antenna element of the first polarization andat least one physical antenna element of the second polarization suchthat the at least one virtual antenna element at least partly utilizethe full antenna aperture; and transmit the signal using the physicalantenna elements connected to the at least one virtual antenna element.6. A network node of claim 5, wherein the first polarization isorthogonal to the second polarization.
 7. The network node of claim 5,wherein connecting each of the at least one virtual antenna elementcomprises connecting each of the at least one virtual antenna element toat least one physical antenna element of the first polarization and atleast one physical antenna element of the second polarization such thatthe at least one virtual antenna element utilize the full antennaaperture.
 8. The network node of claim 5, wherein an angular powerpattern of the at least one virtual antenna element is identical to anangular power pattern of one of the physical antenna elements of theantenna array.
 9. The network node of claim 5, wherein the at least onevirtual antenna element have a total power being the sum of output powerfor all physical antenna elements of the antenna array.
 10. The networknode of claim 5, wherein the at least one virtual antenna element areformed by reduction factors Q and R being applied to all antennaelements of the antenna array.
 11. The network node of claim 10, whereinall non-zero elements of the reduction factors Q and R have constantmodulus.
 12. The network node of claim 10: wherein Q comprises: a firstfactor O_(2A) for reduction along a first dimension and resulting in afirst polarization of the at least one virtual antenna element; and asecond factor Q_(2B) for reduction along the first dimension andresulting in a second polarization of the at least one virtual antennaelement; wherein R comprises: a first factor R_(2A) for reduction alonga second dimension and resulting in the first polarization of the atleast one virtual antenna element; and a second factor R_(2B) forreduction along the second dimension and resulting in the secondpolarization of the at least one virtual antenna element.
 13. Thenetwork node of claim 12, wherein Q_(2A) and Q_(2B) have identicaldimensions resulting in same reduction, and wherein R_(2A) and R_(2B)have identical dimensions resulting in same reduction.
 14. The networknode of claim 12, wherein reduction using Q_(2A) and Q_(2B) andreduction using R_(2A) and R_(2B) result in the at least one virtualantenna element having orthogonal polarization in all directions. 15.The network node of claim 12: wherein elements of Q_(2B) are derivedfrom Q_(2A) by reversing in order, negating, and/or complex conjugatingthe elements of Q_(2A); and wherein elements of R_(2B) are derived fromR_(2A) by reversing in order, negating, and/or complex conjugating theelements of R_(2A).
 16. The network node of claim 5, wherein the atleast one virtual antenna element for each polarization have a phasecenter separation identical to the phase center separation of the fullantenna aperture.
 17. The network node of claim 5, wherein the fullantenna aperture of the antenna array is iteratively reduced at leasttwo times along at least one dimension of the antenna array.
 18. Thenetwork node of claim 5, wherein the connecting each of the at least onevirtual antenna element comprises expanding weight factors applied tothe physical antenna elements by: determining weight factors for thephysical antenna elements responsive to weights applied to the at leastone virtual antenna element utilizing one or more reduction factors Qand R; and applying the weight factors to the physical antenna elementsconnected to the at least one virtual antenna element.
 19. The networknode of claim 18: wherein the at least one virtual antenna element areformed by reduction factors Q and R being applied to all physicalantenna elements of the antenna array; wherein the weight factors areexpanded using the reduction factors Q and R.
 20. The network node ofclaim 5, wherein the partial antenna aperture is used to form one or twobeam ports.
 21. The network node of claim 5, wherein the reducing andthe expanding only are performed along one dimension of the antennaarray.
 22. The method of claim 5, wherein the antenna array is atwo-dimensional antenna array, and wherein the reducing and theexpanding are performed along both dimensions of the antenna array. 23.A method for transmitting a signal using a beamforming antenna array,the antenna array comprising physical antenna elements of a firstpolarization and physical antenna elements of a second polarization, andthe signal requiring use of a partial antenna array of the antennaarray, the partial antenna array requiring less than all of the physicalantenna elements of the antenna array for transmission of the signal,the method being performed by a network node and comprising: forming atleast one virtual antenna element for the partial antenna array tocreate a virtual aperture smaller than the full antenna aperture of theantenna array; connecting each of the at least one virtual antennaelement to at least one physical antenna element of the firstpolarization and at least one physical antenna element of the secondpolarization such that the at least one virtual antenna element at leastpartly utilize the full antenna aperture; and transmitting the signalusing the physical antenna elements connected to the at least onevirtual antenna element.
 24. A method for transmitting a signal using abeamforming antenna array, the method being performed by a wirelessdevice and comprising: obtaining an indication for transmission of thesignal requiring use of less than all of the physical antenna elementsof the antenna array for transmission of the signal; reducing the fullantenna aperture of the antenna array by forming virtual antennaelements for the less than all of the physical antenna elements of theantenna array; connecting each of the virtual antenna elements to atleast part of all physical antenna elements of the antenna array suchthat the virtual antenna elements at least partly utilize the fullantenna aperture; and initiating transmission of the signal using thearray of the virtual antenna elements.
 25. A non-transitory computerreadable recording medium storing a computer program product fortransmitting a signal using a beamforming antenna array, the antennaarray comprising physical antenna elements of a first polarization andphysical antenna elements of a second polarization, and the signalrequiring use of a partial antenna array of the antenna array, thepartial antenna array requiring less than all of the physical antennaelements of the antenna array for transmission of the signal, thecomputer program product comprising software instructions which, whenrun on processing circuitry of a network node, causes the network nodeto: form at least one virtual antenna element for the partial antennaarray to create a virtual aperture smaller than the full antennaaperture of the antenna array; connect each of the at least one virtualantenna element to at least one physical antenna element of the firstpolarization and at least one physical antenna element of the secondpolarization such that the at least one virtual antenna element at leastpartly utilize the full antenna aperture; and transmit the signal usingthe physical antenna elements connected to the at least one virtualantenna element.